Shape memory alloys such as nitinol have been well known since their development by Buehler and Wiley (U.S. Pat. No. 3,174,851) in 1965. Other metals, such as AuCd, FePt3, beta Brass and InTI, exhibit shape memory behavior. These materials have the property of changing shape in response to a change in material temperature. This shape change potential is imparted into the memory metal device through a series of heat treatments.
The transition temperature range is imparted to the material through varying mixtures of intermetallic compounds such as nickel-titanium and heat treatment. The heat treatment methods for the material generally consist of at a minimum high temperature setting of the desired final shape of a device followed by a low temperature straining of the device to a second shape. Then when the device is in the second shape and brought to the transition temperature the device returns to the preprogrammed final shape. The shape change occurs due to the transition of the material from a martensitic to austenitic phase microstructure. These heat-initiated changes cause gross changes in the shape of the implant formed from the memory metal.
Shape memory alloys have been used for a wide range of industrial and medical applications. Medical applications include but are not limited to: catheter, intrauterine contraceptive device, gastrointestinal compression clip, blood vessel filter, coronary artery stent, skin staple, bone staple, and bone plate.
In the prior art memory metal implants have been caused to change shape by heating by their environment, applied current or directed energy. Implants and surgical instruments that change shape in the environment of human body temperature have been described by Jervis and include but are not limited to laprascopic instruments, needle and suture manipulation devices, and expansion and shrinkable coil stents and closures. Implants, that change shape using the Joule effect through resistive heating, have been described by Krumme, Alfidi and Flot. The prior art associated with resistive heating of memory alloys have not recognized the need for the control of the rate of shape change and magnitude of forces applied by the implant to the surrounding tissue. Furthermore, the prior art describes the full transition of the material from martensitic to austinetic microstructure through the delivery of either: 1) a predetermined amount of heat energy to a specific implant (Flot), 2) opening to an initial configuration (Alfidi) or 3) until the shape change breaks contact with current carrying electrodes (Krumme).
Only Jervis recognized the need for force and shape control of a musculoskeletal implant but controlled these through a mechanical actuator that resisted the heat induced shape change. This approach was required due to the transition temperature of the metal fully converting to the austinetic phase at body temperature. In this case, there was no force control of the implant due to the body temperature transition point of the metal, which resulted in the implant applying the maximum potential force to the surrounding tissue.
The prior art is at a significant disadvantage to the subject invention in the field of orthopaedics due to the lack of a method and system to control the rate and maximum shape change force exerted on the surrounding tissue. The clear advantages of the subject invention will be seen in the following review of the prior art.
Methods of heating memory a medical devices to change their shape include: conductive heat transfer (Alfidi [U.S. Pat. No. 3,868,956] and Krumme [U.S. Pat. Nos. 4,550,870 and 4,485,816]), electromagnetic energy heating, and resistive heating using the joule effect (Alfidi [U.S. Pat. No. 3,868,956], Krumme [U.S. Pat. No. 4,485,816] and Flot [U.S. Pat. Nos. 6,268,589 B1 and 6,323,461 B2]). One source of conductive heat energy is the ambient, temperature of the human body (Jervis: U.S. Pat. Nos. 4,665,906, 5,067,957, 5,190,546, and 5,597,378).
Resistive heating has been found to be a convenient method for medical devices. Resistive heating devices have used both AC current (Flot [U.S. Pat. No. 6,268,589 B1]) and DC current (Alfidi [U.S. Pat. No. 3,868,956] and Krumme [U.S. Pat. No. 4,485,816]) to change the implant shape. These systems control the heating current so as to limit the maximum temperature (Flot [U.S. Pat. No. 6,268,589 B1]) and (Alfidi [U.S. Pat. No. 3,868,956]) or extent of shape change of the implant (Krumme [U.S. Pat. No. 4,485,816]). Though these methods and devices control thermal injury to tissue and extent of shape change they are significantly limited in musculoskeletal applications.
Krumme (U.S. Pat. No. 4,485,816; col 6, ln 37-44) controls the maximum temperature and extent of shape change by causing contact between the implant and electrode to break as a result of the shape change. This simultaneous secession of heat or electrical energy flow limits heating of the implant to a level that makes it suitable for use in implant applications. This implant heating strategy results in a predetermined degree of shape change but no control of its force or rate of shape change. This strategy significantly limits implant design because in many musculoskeletal uses solid stable bone structures may not allow the implant to change shape only to provide compressive forces. Thus in this application the shape change would not break contact between the implant and electrode and stop heat energy delivered. Thus the heating device of Krumme can not control either rate of shape change or force exerted on the surrounding tissue.
Alfidi [U.S. Pat. No. 3,868,956, col 7, ln 20-33] provided time and voltage controls to limit the energy applied to an implant so as to fully activate the implant at temperatures compatible with the enclosure of the heating element and the biologic environment in which the implant is used. Alfidi could monitor the actual current flow over a fixed preset time. Alfidi heated quickly to avoid thermal damage [U.S. Pat. No. 3,868,956, col 3, ln 41-43] and expanded the “wire appliance to a desired degree” [U.S. Pat. No. 3,868,956, col 3, ln 10-15] where desired degree was consistently referenced as “it assumes . . . a configuration . . . which . . . is . . . substantially similar to said initial configuration” [U.S. Pat. No. 3,868,956, col 8, ln 61-63]. The initial configuration is the first shape referenced above that is formed during the initial high-temperature heat treatment. Because the implant is fully activated, the heating device of Alfidi cannot control the force exerted on the surrounding tissue.
Flot [U.S. Pat. No. 6,268,589 B1 col 1, ln 44-47] provided voltage control but removed control of the energy delivery time described by Alfidi [U.S. Pat. No. 3,868,956] from the surgeon to lessen the potential for overheating the implant and causing tissue injury. Flot matched an implant size to a specific voltage setting for a fixed period of time through the use of resistor (R17) on the circuit board [U.S. Pat. No. 6,268,589 B1 col 3, ln 21-23]. This matching of implant to voltage required to provide the complete martensitic to austenitic of an implant is proposed by Flot to be matched to an implant mass so that it does not reach a temperatures sufficiently high so as to cause thermal necrosis to surrounding tissue. Flot's approach is limiting in that without control of times the range of implants' mass that can be effected with this invention is limited to 0.8 grams to 2.8 grams [U.S. Pat. No. 6,268,589 B1 col 3, ln 3-7 and col 4, ln. 37-39]. This occurs due to the implant's heating profile being dependent only on applied voltage magnitude and the impedance and mass of the implant. Without user control of both the time and applied voltage the total heating energy to an implant is limited. Thus the implant sizes that can be heated through their transition temperature is limited. The fixed relationship between implant size and control settings presented by Flot teaches against the control of forces exerted by the shape memory alloy implant on the surrounding tissue. Furthermore the inability to control heat energy delivery time teaches away from controlling the rate of memory-metal-implant shape changes so as to protect vital structures.
Jervis, [U.S. Pat. No. 4,665,906 example II and IV] whose medical shape memory alloy implants have a transition temperature substantially at body temperature is the only author of the prior art that realizes the importance of controlling the rate and force applied by the shape changing memory alloy implant. Due to the full transition of martensitic to austenitic microstructure occurring in the implant at body temperature, Jervis controls the shape change with a “mechanical restraint . . . achieving excellent force and time control, and permitting the surgeon to make adjustments as desired.” The advantage of not requiring a separate instrument to control the closure of the implant through applied heat energy overcomes the need for a “mechanical restraint” instrument. This instrument limits the use of the shape memory implant. In medicine a different instrument would be needed for each implant design. Furthermore, many uses would not be realized due to the bulk and functional requirements of the instrument needed to control the closing force and rate of the implant. Finally, once placed and the mechanical restraint removed the implant fully converts to the austinetic microstructure and there is no longer any force control. Thus the invention of Jervis has significant disadvantages compared with the subject invention.
The prior art consistently teaches an instrument or techniques to transform the implant to a single final state. The art describes high temperature setting of an initial shape, low temperature deformation to a second shape state, and heating of the implant to return the implant to its initial shape. The prior art does not present, as the subject invention describes, a device or method to control the martensitic to austinetic transformation so that a plurality of fixation forces and rates can be achieved from each of a plurality of implant designs. Jervis's implant reaches the state associated with body temperature heating. Krumme reaches the state associated with shape change breaking contact with the current source. Alfidi reaches a configuration that is substantially similar to, said initial configuration. Flot provides “predetermined quantities of heat, each corresponding to a given size of clamp” but does not provide a plurality of heat energies to a single style clamp to control the force applied to bone or its rate of closure.
These limitations of the prior art have caused memory metals to be limited in use in orthopaedics. Memory alloy implants have found use as simple two and four leg staples but have not reached the potential of implants that can be manipulated with precise control to change their shape and move bony structures. The lack of surgeon control of forces applied to bone is of significant concern in osteoporotic bone and thus with implants and heat energy sources described in the prior art. Furthermore the quick shape changing movement of implant and bony structures described under the prior art could pinch and injury the spinal cord. This inability to adjust the implants' shape-changing, response within the martensitic to austenitic transformation temperature range to control its force and closure rate has discouraged the clinical use of these systems. The subject invention presents an innovative solution to these clinical issues.